Direction of Arrival (DOA) estimation is an important topic in acoustic signal processing. Estimating the location of acoustic sources is important in many applications such as teleconferencing, camera steering, and spatial audio, to name a few. In the vast majority of known techniques for DOA estimation, the free-field assumption is required to hold, that is that there are no reflections introduced from the environment, or at least the direct sound from the source to the sensor array is dominant over the reverberant path.
But in real acoustic environments, a transmitted signal is often received via multiple paths due to reflection, diffraction, and scattering by objects in the transmission medium. This multipath effect can be understood as mirror-image sources which produce multiple wavefronts interfering with each other, a fact that unfavorably affects direct-path localization techniques. The image sources tend to widen the estimated Direction of Arrival (DOA) distributions around the true DOA, an effect that grows in proportion to the reverberation time of the acoustic environment.
DOA estimation and localization in reverberant rooms is still possible to some degree, if it can be assumed that the energy of the direct wavefronts predominates over the contributions of early reflections, reverberations, and noise. The performance can be improved to some extent by pre-selecting the signal portions that are less severely distorted with multipath signals and noise, as well as signal portions where one source is significantly more dominant than others. On the other hand, a propagation model may be employed that takes into account some of the early reflections introduced by the acoustic environment. It has been shown that single reflections may convey additional information which can be exploited to not only make sound-source localization possible in reverberant rooms, but also to extract additional important spatial information regarding the sound sources. For example, the additional information may be exploited to make range and elevation estimates, something that would not be possible with a sensor array under free-field conditions.
Early reflections may have an adverse effect on the performance of several applications related to microphone-array signal processing. For example, when a microphone array is close to one of the walls of a room, the reflection introduced by that wall may significantly degrade the performance of a DOA estimation. Yet as a practical matter, placing a microphone array far away from the walls in a room may be difficult or impossible.